When cellular phones or mobile radio devices are manufactured, they must be calibrated to transmit a known RF power (galvanic power) from the transmitter to the antenna structure as well as to radiate known RF power (EIRP/ERP) from the antenna structure. The power calibration also must be performed at various levels throughout the operating range of the mobile device. This calibration ensures that the highest power transmitted to and from the antenna produces legal and acceptable specific absorption rate (SAR), which protects the health and safety of the user, and for a given EIRP/ERP limit. As well, the power calibration assists in maintaining a valid wireless link in cellular communications while minimizing power usage, thereby extending battery life, and maximizing coverage and capacity of the cell sector.
Conventionally, a sample of every cell phone model to be retail marketed is tested for the maximum EIRP/ERP Level in a test lab for several hours, with a considerably large measurement uncertainty of greater than 2.0 dB. Before performing this test, the galvanic power of the cell phone must be calibrated and the cell phone is set to radiate with maximum galvanic power.
The cell phone RF power is conventionally delivered to the cell phone test set using a physical hardwired connector just before the antenna section of the RF circuit, and is adjusted via a cable connection between the RF connector on the cell phone and test set. Once the maximum EIRP/ERP level is adjusted or found to meet regulatory limits for a given galvanic power, then only the SAR level measurements are performed for legal compliance.
To calibrate the RF power of a cell phone having more than one antenna, as well as for devices with MIMO architecture, the manufacturer usually provides a single RF connector along with an RF switch, filter and impedance matching for each antenna circuit. As the RF connector is well before the RF switch, filter and matching circuits, the performance of each of the antenna circuit is unknown even after successfully completing all the manufacturing tests of the cell phone using the conventional method.
While performing the SAR measurement, the maximum galvanic power level obtained in the first step is used as the starting level. If the galvanic power requires adjustment to meet the SAR limits, the adjusted galvanic power level will be considered the maximum power that can be fed to the antenna, and then EIRP/ERP levels must be re-evaluated.
All the manufactured cell phone samples of the same model are calibrated using the new galvanic power level as the maximum power to the antenna. Once this maximum level is calibrated, up to 20 intermediate power levels are set throughout the dynamic range. In order to perform these tests, a galvanic RF link is established using a cable between the cell phone RF connector and the test set. The RF connector of the cable for the cell phone connection end wears out over time and is replaced based on an estimated maximum number of insertions during the manufacturing test cycle for all produced units (usually very large). Production testing is stopped and a new cable must be introduced and a recalibrated before manufacturing testing can resume. This introduces delay and cost.
After each cell phone is calibrated for appropriate RF power levels (galvanic) in order to meet the legal EIRP/ERP as well as SAR levels, each cell phone is further tested for Tx and Rx performance. To perform this test, the cell phone is connected to the cell phone tester using an RF cable between its RF connector and test equipment as discussed above. In the majority of the cases, the RF power calibration is done in one location and the Tx/Rx parametric testing is done in another location. In the event these tests are performed at different locations, the RF cable connected between the cell phone and test set must be replaced frequently with a new RF cable due to the large number of insertions. A recalibration of the RF cable must be performed before continuing the manufacturing Tx/Rx parametric testing of cell phones which introduces further delay and cost.
During board level manufacturing or designer testing to optimize the RF parameters of cell phones, the measurements are performed with an RF galvanic connection. This method does not provide all the necessary measurements to understand the complete performance of the RF circuit.
During the design and development of new cell phone models, designers often go though a series of iterations to improve the radiated performance of the cell phone antenna model for achieving greater usable range while targeting low SAR levels and low galvanic RF power. Each time the radiated performance of the cell phone is measured, it is necessary to go to the test labs where EIRP/ERP levels can be optimized through a series of measurements. Currently no tool exists for finding accurate near field spatial distribution of the RF radiation in the near field to minimize unwanted radiation. Designers rely on the conventional testing methods in the test labs for far field radiated patterns and then debug at the circuit board level, which is a very tedious and complicated process.
For measuring antenna properties such as radiation pattern, gain, and directivity, near field scanners are employed to gather accurate amplitude and phase data and subsequently to calculate the equivalent far field value using one of many transformations known and available in the prior art. To accurately estimate the far field, those skilled in the art believe the measurement distance between the probe and antenna under test should be greater than or equal to one wavelength. Current near field testing is performed using a mechanical scanner with a single compensated probe which can detect both polarizations. These measurements usually take more than a few hours to complete a scan of the entire radiating surface.
When near field radiation is measured, the array elements and ground plane behind them have considerable effect on the near field distribution of the radiating source as well as its far field properties. In the prior art, using multi-axis near field measurement systems, the measurement is performed at greater than one wave length from the array surface in order to minimize the ground plane effect, which is then accounted for relatively easily. Alternatively, a perfect near field absorber such as that described in U.S. Pat. No. 6,762,726 B2, issued Jul. 13, 2004, is used to increase the isolation between the radiating and array surfaces thus decreasing the mutual coupling. In either case, the array sensitivity is significantly decreased.
Therefore, there is a need in the art for method and apparatus of measuring EIRP and ERP from RF and microwave transmitters, particularly with an application to MIMO mobile or cellular phone measurement and calibration of EIRP/ERP, as well as Tx/Rx performance. Such apparatus should preferably perform measurements in the near field, without the need for a physical galvanic RF connection, and without the need for an absorber.